1. Field of the Invention
The present invention relates to an optical fiber amplifier and more particularly to a gain flattening filter used in a gain flattened optical fiber amplifier.
2. Description of the Related Art
As the recent increase in the quantity of data in geometrical progression requires expansion of the transmission band width of the wavelength division multiplexing optical communication system, it has been widely accepted now that the wavelength division multiplexing optical communication system must be equipped with a gain flattened optical fiber amplifier. A gain flattening filter employed in the gain flattened optical fiber amplifier may be constructed using either active elements or passive elements. Among these two, the utilization of passive elements is accepted more. As passive elements, optical fiber gratings or dielectric thin film filters are typically deployed. However, the gain flattening filters employing the optical fiber gratings are not reliable under variable environmental factors, such as temperature and moisture, thus dielectric thin film filters are a better choice to be used in the gain flattening filter.
According to the location of the gain flattening filter in the optical power amplifier, the gain flattening filters can be classified into two types: end filters and midway filters. An end filter is located at the rear end of the amplifying medium, while a midway filter is located at the middle portion of the amplifying element. The end filter has a drawback in that it tend to reduce as much gain of the optical fiber amplifier as its own insertion loss, while having an advantage in that it can be designed to limit the peak loss to within 5 dB. In contrast, the midway filter has an advantage in that it compensates for the basic insertion loss of the midway filter in the amplifying element, while having a disadvantage in that it requires a peak loss of at least 7 dB in the design process thereof. There is a limitation in utilizing the dielectric thin film filter as a midway filter as it is difficult to reach a peak loss of at least 6 dB due to the difficulty in its coating process.
FIG. 1 is a schematic view illustrating the configuration of a conventional gain flattened, optical fiber amplifier. FIG. 2 is a graphical illustration depicting a loss curve of a first or second gain flattening filter employed in the gain flattened optical fiber amplifier shown in FIG. 1. As shown in FIG. 1, the conventional optical fiber amplifier includes first to third isolators 110, 150, and 210, first and second pumping light sources 120 and 190, first and second wavelength selective couplers 130 and 200, first and second erbium-doped optical fibers 140 and 180, and first and second gain flattening filters 160 and 170.
In operation, the first isolator 110 allows an optical signal inputted to the optical fiber amplifier to pass intact through the first isolator 110, while intercepting a light inputted in a direction opposite to that of the optical signal—that is, a light inputted from the first wavelength selective coupler 130. The first wavelength selective coupler 130 combines the optical signal inputted from the first isolator 110 and a pumping light inputted from the first pumping light source 120, then outputs the combined optical signal to the first erbium-doped optical fiber 140. The first pumping light source 120 pumps the first erbium-doped optical fiber 140 by exciting erbium ions in the first erbium-doped optical fiber 140. A laser diode capable of outputting a pumping light may be utilized as the first pumping light source 120. The first erbium-doped optical fiber 140 is pumped by the pumping light inputted through the first wavelength selective coupler 130, then amplifies and outputs the optical signal inputted through the first wavelength selective coupler 130.
The second isolator 150 allows the optical signal inputted through the first erbium-doped optical fiber 140 to pass intact through the second isolator 150, while intercepting light inputted in a direction opposite to that of the optical signal. The first and second gain flattening filters 160 and 170 are midway filters and serve to sequentially flatten the gain of the optical signal inputted through the second isolator 150. Each of the first and second gain flattening filters 160 and 170 includes a dielectric thin film filter. Referring to FIG. 2, note that the peak loss is about 5 dB, as indicated by the loss curve of the first gain flattening filter 160 or the second gain flattening filter 170. As the gain flattened, optical fiber amplifier requires a peak loss of about 10 dB, the first and second gain flattening filters 160 and 170 each having a loss peak of 5 dB are connected in series with each other in the gain flattened optical fiber amplifier.
The second erbium-doped optical fiber 180 is pumped by a pumping light inputted through the second wavelength selective coupler 200, then amplifies and outputs the optical signal inputted from the second wavelength selective coupler 200. The second wavelength selective coupler 200 outputs the optical signal inputted from the second pumping light source 190 to the second erbium-doped optical fiber 180 and outputs the optical signal inputted from the second erbium-doped optical fiber 180 to the third isolator 210. The third isolator 210 allows the optical signal inputted through the second wavelength selective coupler 200 to pass intact through the third isolator 210, while intercepting a light inputted in a direction opposite to that of the optical signal.
As described above, in the case where the gain flattened optical fiber amplifier requires a peak loss exceeding the limit of each dielectric thin film filter, a plurality of gain flattening filters, each including a dielectric thin film filter, must be connected in series with each other in the conventional gain flattened, optical fiber amplifier, thereby increasing the volume and manufacturing cost of the entire gain flattened optical fiber amplifier.